In order to fabricate semiconductor wafers with submicron features using etch and deposition processes, modem semiconductor processing systems utilize plasma assisted techniques such as reactive ion etching (RIE), plasma enhanced chemical vapor deposition (PECVD), sputtering, reactive sputtering, and ion assisted physical vapor deposition (PVD). In addition to the above-referenced co-pending applications, another example of a gas plasma processing system is described in U.S. Pat. No. 5,234,529, to Wayne L. Johnson, the inventor of the present application. In such known systems, a gas is introduced to a processing environment wherein a gas plasma is formed and maintained through the application of radio frequency (RF) power. Typically, RF power is inductively coupled to the plasma using a helical coil.
Normally, the generation of a gas plasma also produces a substantial amount of heat that must be removed in order to maintain the processing system at a process-specific temperature. The removal of this heat has heretofore been inefficient and based on a cumbersome design. Known ESRF plasma sources have been cooled using baths of liquid coolants, such as FLUORINERT Electronic Liquid, which also act as dielectrics. The definition of a good dielectric at radio frequencies is that the fluid must have a low power loss per unit volume when exposed to an intense electric field. However, these particular fluids disadvantageously adsorb large quantities of gas, such as air. In other words, liquid (coolant) surfaces exposed to air (and other gases) adsorb the gas into the surface layer. This adsorbed gas is then dispersed throughout the liquid (volume or interior) when the liquid is mixed (i.e., moving or being pumped through the cooling system). Furthermore, gas may be adsorbed into the coolant after the coolant pumps are stopped. When the pumps stop, if coolant in the high parts of the system drains to lower parts, then air replaces the drained coolant. When the pumps are restarted, the air may be broken down into bubbles which become another source of adsorbable gas.
In the high field regions, strong dissipation can occur leading to high local heating, hence, raising the local temperature of the coolant fluid. In so doing, the rate of gas evolution is increased permitting more gas to come out of solution, and generate bubbles that coalesce on the coil surface attracted by dielectro-fluoretic attraction. The attached bubbles generate a dielectric difference at the coil surface which leads to enhanced non-uniform electric fields, localized heating, and arcing. This arcing can occur at voltages well below the measured dielectric strength of the fluid if the gas is not evolved from the liquid coolant before use in the resonator cavity. For example, FLUORINERT Electronic Liquid adsorbs a volume of gas equivalent to its own liquid volume and must be treated to remove the trapped gas.
In order to avoid arcing due to the rapid evolution of adsorbed gas, known systems gradually increase power to the plasma source while continuously pumping coolant through the ESRF plasma chamber. The gradual increase in RF power takes place over a period of time sufficient to slowly evolve adsorbed gas from the coolant. Although running the coolant in this way evolves trapped gases, a considerable amount of time is required. Often this process will take hours, thereby delaying the use of the plasma system.
In addition to the lengthy time period required by known systems to evolve adsorbed gas, the cooling systems coupled to a plasma source may be very cumbersome due to the large cooling lines used in large wafer (i.e., 300 rnm) processing systems. Consequently, significant amount of air is generally adsorbed when the processing chamber has been opened with the coolant lines remaining attached. The lines have typically remained attached since the coolant lines may contain hundreds of pounds of coolant. As a result, lifting the attached lines to open the chamber has been difficult, but not impossible.
Previously, it was not known how to replace the large lines with an alternate cooling mechanism. The large lines were required in order to provide the large coolant exchange (e.g., approximately 50-75 gallons/minute) needed to remove the heat from the process tube. Also, flexible lines were difficult or impossible to use because of the weight and pressure of the coolant required.
It is an object of the present invention to provide an improved method and system for cooling an ESRF source that reduces a delay when conditioning a dielectric fluid.
It is another object of the present invention to reduce the conditioning delay while performing at least one of the following tasks: (1) increasing the reliability of the liquid coolant as a dielectric, (2) increasing the plasma density, and (3) increasing the starting range of the coolant. The latter two tasks relate to alleviating the stringent conditions placed upon the application of RF power to the plasma source in order to avoid failure of the coolant and, hence, premature coil arcing. By conditioning the coolant to avoid premature arcing, the RF plasma source may be run at nominal power (instead of some reduced power level). Therefore, a nominal plasma density may be obtained, and robust start conditions can be achieved without significant delay (due to ramping the RF power).
It is a further object of the present invention to reduce the conditioning delay while decreasing the number, frequency, or severity of cavity failures, i.e., coil arcing.
These and other objects of the present invention are achieved according to a method and system of degassing a liquid coolant used in an RF plasma source-based processing system. By utilizing a vacuum to remove the gas that has been adsorbed into the liquid coolant, the system reduces arcing caused by micro-bubbles attached to the surface of a submerged coil.